Recently, energy storage technologies are receiving continuously increasing attentions. As application areas expand to energies for mobile phones, camcorders and notebook PCs and further to electric vehicles, more systematic efforts are made for the researches and developments of electrochemical devices. In this aspect, electrochemical device area is in the center of the attentions, and development of chargeable and dischargeable secondary batteries is becoming a focus of attention. Recently, in developing such batteries, research and development to design new electrodes and batteries are under way in order to improve capacity density and specific energy.
Among the currently-available secondary batteries, lithium secondary battery developed in the early 1990s is coming into spotlight because of advantages such as higher operating voltage and superior energy density compared to conventional batteries such as Ni-MH, Ni—Cd, lead-sulfate batteries, and the like that use aqueous solution electrolyte. However, lithium ion secondary battery among the secondary batteries mentioned above has disadvantages of safety issues such as ignition and explosion, which may occur due to use of organic electrolyte, and complicated preparation thereof. The lithium-ion polymer secondary battery has recently been suggested as one of next-generation batteries as an improvement to the disadvantages of the lithium ion secondary battery mentioned above. However, relatively lower battery capacity than the lithium ion secondary battery, and particularly, insufficient discharge capacity at low temperature require immediate improvements.
There are increasing demands for high-capacity anode material in order to address the above issues, and metals (metalloids) such as Si, Sn, or the like with high theoretical capacity are applied as the anode active materials. However, such anode active materials provide negative influence on performance and safety of the battery due to deteriorating cycle characteristic and excessive volume expansion with repeated charging and discharging. Accordingly, studies have been conducted to find ways to improve cycle characteristic and alleviate volume expansion, using metal (or metalloid) oxides such as silicon oxide (SiOx), and so on. However, the metal (or metalloid) oxide has an irreversible phase formed from the initial reaction of oxygen and lithium upon lithium intercalation, which leads to very low initial efficiency.
In order to compensate this, the metal (or metalloid) oxide can be alloyed with lithium in advance in order for the metal (or metalloid) oxide to contain lithium, from which irreversible phases such as lithium oxide, lithium metal oxide and the like are less generated in the initial charging and discharging of the battery, thereby increasing the initial efficiency of the anode active material.
However, there are problems that a by-product of lithium may be generated during reaction of the metal (or metalloid) oxide and a lithium source and remain on the surface of the composite, which lead to the increase of pH in a water-based binder system containing the by-product, making it difficult to mix an anode active material slurry. This alters the properties of a binder present in the slurry, thereby rendering the adhesion of an electrode weak.